The power supply in a PC is a complex device using switch mode circuitry and transformers to generate +and- 12v, for modems and communications ports. +and-5volts for the… logic circuitry,3volts for the processor. All these supplies are generated and electronically regulated. It is such a waste when they are thrown a way still working. They are mini masterpieces of electronic engineering and are capable of starting a car!

It's a power supply with a positive and negative voltage, where one of the voltages tracks the other. For example: the negative voltage tracks the positive. If you regulate th…e positive to 15v, the negative is going to be -15v. This allows very precise symmetric power.